1. Field of the Invention
The present invention relates to a storage system and a storage system control method.
2. Description of the Related Art
In various institutions such as governments, business enterprises, and universities, for example, data are managed using comparatively large-scale storage systems so that large amounts of various types of data can be handled. This type of storage system is constituted by a disk array apparatus or the like, for example. A disk array apparatus is constituted by a large number of storage devices provided in array form, and provides a storage area based on a RAID (Redundant Array of Independent Disks), for example. At least one or more logical volumes are formed on a physical storage area provided by a storage device group, and the logical volume is provided to a host computer (more specifically, a database program running on the host computer). The host computer (to be abbreviated to “host” hereafter) is able to perform data writing and reading to and from the logical volume by transmitting predetermined commands.
The amount of data to be managed in these various institutions increases by the day. In anticipation of a future increase in demand, providing storage systems with a large amount of storage areas in advance is currently being considered. However, the unit price of storage devices is decreasing year by year, and hence there are fears that the prior investment required to purchase a large amount of storage devices in preparation for a future increase in demand is excessive. The cost of the storage system is reduced by instead purchasing storage devices appropriately when a new storage area becomes necessary.
Conversely, when storage areas are only provided in accordance with current needs, it is impossible to respond immediately to new demand, and as a result the convenience of the storage system decreases. Hence there is a trade-off between cost and capacity to respond to change. In response, a technique has been proposed for managing the storage areas of each storage device existing in a storage system as virtual storage areas, releasing unused storage areas assigned to the host, and assigning these storage areas to another host (Japanese Unexamined Patent Application Publication 2005-31929).
With the technique described in this document, an unused storage area occupied by a certain host can be released forcibly and assigned to another host requiring a new storage area. In so doing, the storage resources of the storage system can be used effectively.
However, even when a host requires a new storage area, it does not always require the entire storage area immediately. Data are stored in the storage area gradually as the data processing service performed on the host progresses. In other words, the amount of data stored in the storage area is small in the initial stage and increases steadily over time. Hence, with the technique described in the above document, it is impossible to eliminate storage area waste in the initial stage.